1. Field of the Invention
The present invention relates generally to a method for determining the location of implanted medical devices, and more particularly to locators incorporated into a drug port to allow determination of septum orientation prior to injection.
2. Description of Related Art
Medical devices implanted beneath a patient's skin are well known in the art. Many devices require locating systems so that their exact location can be determined using various techniques such as ultrasound and fluoroscopy. There is a wide array of patents utilizing locating techniques for catheters, injection ports and other medical prosthetics. For example, U.S. Pat. No. 4,697,595 to Breyer et al. discloses a locating technique for catheters. A catheter with embedded ultrasonic piezoelectric transducers is used. The transducers connect to electrical leads that relay an electrical signal in response to an ultrasonic beam, thereby pin-pointing the position of the catheter tip. Locators have also been used for implantable prosthetic devices.
U.S. Pat. No. 5,405,402 to Dye et al. discloses a radiographic marker incorporated into an implantable prosthesis. The marker comprises a tension coil spring that forms a continuous ring to encircle the prosthetic member. The prosthetic member can then be readily aligned using the radiographic marker.
The present invention relates generally to implantable medical devices and more specifically to devices containing self-sealing injection sites or ports. Devices such as tissue expanders and drug delivery access ports feature self-sealing septa so the devices can be filled periodically by needle injection. It is vital to accurately determine the location of such septa before injecting a drug that could cause considerable damage if inadvertently injected into the peripheral tissues of the patient. Many patents have attempted to solve this vexing problem. U.S. Pat. No. 4,781,685 to Lehmann et al. addresses the problem by providing an external signaling device which signals when a needle is positioned correctly inside of an implantable drug dispensing capsule. U.S. Pat. No. 5,137,529 to Watson et al. addresses the locating problem by placing a barium impregnated silicone tube over an outlet connector to provide a radiopaque indicator of the location of the injection port. U.S. Pat. No. 5,725,507 to Petrick also uses barium sulfate to provide a radiopaque member for location of an injection site. Finally, U.S. Pat. No. 5,201,715 to Masters discloses an additional way to locate an injection port by using implantable devices having distinct ultrasonic echographic signatures. The ultrasonic targets are placed below the septum within the injection port, and when an ultrasonic source irradiates the target, multiple reflections occur from the targets.
Unfortunately, it is sometimes very difficult to determine the precise location and orientation of injection ports. The prior art discussed above, while addressing the problem of determining the location of an injection site, does not really solve the problem of also determining orientation of the device within the body. While locating the injection port itself is important, mistakes can be made despite using the above-mentioned two-dimensional location techniques. For example, although the injection site in Petrick can seemingly be located by its radiopaque marker, the orientation of the drug port is not revealed. Thus, if the port is actually inverted (i.e., the septum faces away from the skin surface), the needle could be pushed into the back of the port causing numerous problems ranging from puncture of the port to injection of the drug into the peripheral tissues. Additionally, if the back of the port is sufficiently hard, a portion of the needle could break off inside the body causing infection and other complications. In either case, invasive surgery is necessary to remove the damaged items, opening up the possibility for even further complications.
The prior art mentioned also exhibits certain other shortcomings. In particular, although Watson attempts to address the problem of orientation, the placement of the barium-impregnated tube doesn't reveal whether the port is correctly oriented for injection or whether it has "flipped" over. A problem with Lehmann et al. is the device's complexity and the fact that although the device signals when the needle is in the correct position, it does nothing to aid the physician in locating the port and also does not reveal orientation in advance of the attempted injection. Masters describes a method to locate the center of an injection port by providing a locator that produces ultrasonic signatures. This method does not appear to reveal septum orientation.
Thus, it would be desirable to create an implanted drug port with a self-sealing injection site equipped with means for determining location, not only in terms of position inside of the body but also in terms of orientation at that site. In addition, it would be desirable to provide a simple, easy and inexpensive way to create this apparatus. It would be further desirable to provide this apparatus so that current medical techniques for viewing an implanted device could be employed. Further, the locating means should not interfere with widely used diagnostic imaging techniques such as MRI (Magnetic Resonance Imaging).